code: def conllstr2tree(s, chunk_types=("NP", "PP", "VP"), root_label="S"):
      """
      Return a chunk structure for a single sentence
      encoded in the given CONLL 2000 style string.
      This function converts a CoNLL IOB string into a tree.
      It uses the specified chunk types
      (defaults to NP, PP and VP), and creates a tree rooted at a node
      labeled S (by default).
  
      :param s: The CoNLL string to be converted.
      :type s: str
      :param chunk_types: The chunk types to be converted.
      :type chunk_types: tuple
      :param root_label: The node label to use for the root.
      :type root_label: str
      :rtype: Tree
      """
  
      stack = [Tree(root_label, [])]
  
      for lineno, line in enumerate(s.split("\n")):
          if not line.strip():
              continue
  
          # Decode the line.
          match = _LINE_RE.match(line)
          if match is None:
              raise ValueError(f"Error on line {lineno:d}")
          (word, tag, state, chunk_type) = match.groups()
  
          # If it's a chunk type we don't care about, treat it as O.
          if chunk_types is not None and chunk_type not in chunk_types:
              state = "O"
  
          # For "Begin"/"Outside", finish any completed chunks -
          # also do so for "Inside" which don't match the previous token.
          mismatch_I = state == "I" and chunk_type != stack[-1].label()
          if state in "BO" or mismatch_I:
              if len(stack) == 2:
                  stack.pop()
  
          # For "Begin", start a new chunk.
          if state == "B" or mismatch_I:
              chunk = Tree(chunk_type, [])
              stack[-1].append(chunk)
              stack.append(chunk)
  
          # Add the new word token.
          stack[-1].append((word, tag))
  
      return stack[0]
  
imports: import re
  from nltk.metrics import accuracy
  from nltk.tag.mapping import map_tag
  from nltk.tag.util import str2tuple
  from nltk.tree import Tree
label_desc: Use image node_id 4 for calling a global function with example usage: conllstr2tree(s, chunk_types, root_label) and returns: stack